COO

'Coo' may be:

★ A noun and verb denoting the call of pigeons.

★ An interjection of surprise and admiration.

★ A derivative of the word "cool"; it has a similar usage but tends to be more sincere where "cool" is often used in a dispassionate manner as mere acknowledgement.
or refer to:

★ Coo, a municipality in the Belgian province of Liège
Also see COO, as an acronym.